This invention relates to a method of manufacturing new and improved noise attenuating panels and more particularly to a honeycomb panel having an imperforate sheet on one surface and a perforate facing sheet with a layer of porous material bonded thereto wherein there is continuous communication between the core cells and the atmosphere adjacent to the noise to be attenuated. The material is specifically suitable for use in a severe environment such as within an aircraft engine.
In manufacturing sound suppressive metal honeycomb sandwich panels which are exposed to extreme environment and the sound produced by modern turbine aircraft engines, it is common practice to provide a cellular structure utilizing the Helmholtz resonance principle wherein a first imperforate sheet of material is bonded to one core surface of a sheet of cellular core material and a thin perforated sheet of like material is applied to opposite core surface.
Panels of this type construction, although satisfactory for some sound frequencies are found to be an inefficient noise attenuator over a broad range of frequencies customarily encountered in and around aircraft jet engines. Additionally it has been found that the perforations of the perforated sheet exposed directly to gas flow within the engine or outer aircraft surfaces create turbulence to high speed flow as, for example, within the throat of a fan type jet engine or along exterior aircraft structure during normal flight conditions.
Other concepts have included interposing a sheet of fibrous material between the perforated sheet and the core. This has proven to be unsound structurally in severe engine environments.
Attempts to successfully manufacture this and various other sound suppression material of this general type have resulted in the adhesive used for bonding of the various components to the core to ooze or wick into the perforations and at least partially fill those perforations, changing the flow resistance between the atmosphere where the sound is produced and the core cells. When the perforations are made large enough to accommodate the excess adhesive, the structural strength is reduced and the air flow turbulence is increased. In those structures where the porous fibrous material is inserted between the cellular core and the outer perforate sheet, the adhesive wicks into the pores of the porous fibrous material as well as into the perforations, thus further reducing the effectiveness of the resulting structure.